Heretofore, petroleum liquids have generally been analyzed in the laboratory, or on site in a laboratory cabin which reproduces laboratory conditions. The liquid is initially separated into a light portion and a heavy portion by means of a distillation apparatus. The light portion is analyzed by means of a gas chromatograph, whereas only two or three overall physical measurements are used to characterize the heavy portion (e.g. molar mass, density, . . . ). This method suffers from the drawback of requiring additional apparatus (for distillation) which is difficult and lengthy to put into operation. Further, the operations of recovering and measuring the various products may give rise to additional sources of error. In some laboratories, the liquid is injected directly into a gas chromatograph whose circuit comprises an injector, a short duct, and an analysis column. Quantitative analysis is performed by means of the method using an internal standard. This method makes it possible to avoid prior separation, but the system becomes polluted very quickly if heavy oils are injected therein, since the intermediate components migrate slowly but irremediably along the short duct and thus reach the analysis column.
In order to overcome this drawback, it has been proposed in U.S. Pat. No 3,030,798 to Lichtenfels a two-stage process using two packed columns connected in series. In this known technique, the fluid mixture to be analyzed is initially injected into the first column and carrier gas is flowed through the first and the second column. When all the components of interest i.e. the lighter components have emerged from the first column, the direction of flow of the carrier gas in the first column is reversed while it remains unchanged in the second column. The effect is to remove from the first column the heavier components which remain therein, and once this backflushing is completed, the first column is in condition for receiving another charge to be analyzed. At the same time, the analysis continues in the second column.
The system described in this patent is disadvantageous in that the flow of carrier gas is controlled by valves which are part of the circuit through which the substance to be analyzed is circulated. This may give rise to leaks and creates "lost volumes" detrimental to the accuracy of the analysis.
The aim of the present invention is to make it possible to analyze the components of a sample of crude oil stored under atmospheric conditions. This analysis should be capable of being performed on a drilling site using portable equipment and without taking special precautions, and it should be possible to analyze a large number of samples in succession without the need for any maintenance operation. High accuracy should be achieved concerning the lighter portion of the liquid (up to C.sub.6 components). No restriction should exist with regard to the type of crude oil capable of being analyzed.